1. Field of the Invention This invention relates to heating elements for semiconductor bonding tools. More particularly the invention relates to a novel heating element which comprises a conductive-resistive film applied directly to bonding tools of the prior art type.
2. Description of the Prior Art
Wire bonding and die bonding tools are well known in the prior art. It is also known that heating these bonding tools during certain types of bonding operations produces better bonds which are completed faster. The heating elements for such prior art bonding tools generally fall into one of two types. Either the tool holder has a heating element embodied therein or the heating element was attached to the bonding tool proper. Heretofore, cartridge type heaters and wound wire heaters had to be large in comparison to the size of the bonding tool because the heat was transmitted through one or more air to heater surfaces. Attempts to silver solder or attach the prior art heaters to the binding tool or bonding tool holder did not succeed because premature failure of the heating element rendered the expensive bonding tool or holder inoperable thus requiring expensive rework.
Prior art heating elements for bonding tools required a substantial amount of heat because the heating elements and/or heat sink tool holders have large areas for heat dissipation. Also, the temperature of the tool holder approaches the temperature of the heating elements.
Cartridge and wire wound heaters generate their heat at a point which is remote from the working tip of the bonding tool. This requires that the heater not only supply additional heat which is dissipated, but that the heater be operated at a substantially higher temperature to overcome the temperature gradient between the heater and the working tip.
Heretofore, it was known that resistor materials could be deposited on dielectric substrates. Such prior art resistors have been made by depositing special paste through slots or silk screens, by evaporating deposits, by sputtering deposits or electroplating deposits. Prior art deposited resistors are designed to hold their ohmic resistance over a narrow range of temperatures, usually 40.degree. C. to 50.degree. C. In order to maintain desired temperature stability, such resistors are provided with large areas for heat dissipation. Such resistors may provide up to one square inch of surface for each watt of heat generated.
Bonding tools are known in the prior art which are heated by passing electrical current through the tool. These resistance type bonding tools were generally used to heat metal capilliaries or to bond preformed electrical leads or wires to terminal pads on a foil pattern or a substrate. Such tools may be employed on electrodes or pads on a semiconductor device if care is taken to avoid damage to the semiconductor device proper.
Prior art heating devices were large and often obstructed part of the view of the workpiece when the bonding tool was in or approaching the working position. The large mass of the prior art heating elements required larger structural machine parts and drives or alternatively resulted in slower operation of the bonding machines than could be obtained without heaters.
It would be desirable to retain all of the advantages of presently knon bonding tools and heated bonding tools without incurring the disadvantages of having to employ prior art heating devices.